Radar can be used to measure range and direction to a reflective object, which is often referred to as a “target”. However, a return may be achieved from all possible reflecting surfaces, which in the case of a large structure such as a ship or oil rig can give a large return signal, of extended angular range and a range of distances. This is generally acceptable for standard collision avoidance applications.
However such diverse returns of the radar signal are not useful if it is desired to use radar to measure distance to a target, such as a vessel position relative to an oil rig, to a few meters or less.
It is known to provide active radar reflectors. An example of such a reflector is the “See me” (trademark) radar target enhancer which detects when a vessel carrying the target enhancer is illuminated by a radar system. It amplifies the incoming signal and retransmits it, and in so doing synthesises a radar cross section of around 34 square meters.
However a signal propagating between a radar and an active target may be able to propagate by several signal paths. A particularly problematic scenario is where part of the signal undergoes “line of sight”, i.e. direct, propagation and another part of the signal undergoes reflection, for example from the surface of the sea. These two signal paths can interfere, either constructively or destructively depending on the relative path lengths. For X band radar the wavelength of the radar is around 3 cm or so and consequently rolling of a ship carrying the target and/or wave height and position variations can give rise to short periods of destructive interference.